The present invention generally relates to a method for producing a semiconductor device having a silicon-on-insulator (SOI) structure, and in particular, to a method for producing a semiconductor device having a bulk element region and a silicon-on-insulator element region.
A semiconductor device having a bulk element region and a silicon-on-insulator element region, or a bulk-SOI hybrid integrated circuit device has advantages of excellent crystallization of a bulk and excellent element insulation of the silicon-on-insulator (SOI) structure. For example, logic circuits are formed in bulk element regions in a semiconductor substrate, and output circuits having a high-voltage withstanding output driving force are formed in SOI element regions formed on the semiconductor substrate.
In a conventional method for producing such a semiconductor device, a local oxidation of silicon (hereafter simply referred to as a LOCOS) process is carried out two times. First, an insulation film is formed which is used for electrically insulating bulk element regions from each other, and secondly another insulation film is formed which is used for electrically insulating SOI element regions from each other and for electrically insulating the SOI element regions from the bulk substrate. Additionally, since the LOCOS process is based on thermal oxidation, a long time is taken to form an insulation film for element insulation. As a result, the production yield of the conventional method is low.
Generally, an underlying insulation layer on which a polysilicon layer of an active layer is formed, is formed by the thermal oxidation process. By this process, it is actually difficult to form a thick underlying insulation film. Generally, there is a limitation of thickness up to approximately 1 to 2 .mu.m. With the thickness of the underlying insulation layer equal to approximately 1 .mu.m, it is found that the polysilicon layer on the underlying insulation layer may be destroyed when a voltage of an approximately 200 volts is applied between the semiconductor substrate and a drain formed in the polysilicon layer on the underlying insulation film. From this viewpoint, it is required to form the underlying insulation layer as thick as possible.